1. Field of the Invention
The present invention relates generally to electronic memory devices and, more particularly, to a device and method for increasing security of data stored in memory devices.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Microprocessor-controlled circuits are used in a wide variety of applications. Such applications include personal computers, control systems, telephone networks, and a host of other consumer products. A personal computer or control system includes various components, such as microprocessors, that handle different functions for the system. By combining these components, various consumer products and systems may be designed to meet specific needs. Microprocessors are essentially generic devices that perform specific functions under the control of software programs. These software programs are generally stored in one or more memory devices that are coupled to the microprocessor and/or other peripherals.
Semiconductor memory devices, such as dynamic random access memory (DRAM) devices, are widely used for storing data in systems such as computer systems. A DRAM memory cell typically includes an access device such as a field effect transistor (FET) coupled to a storage device such as a capacitor. The access device allows the transfer of charged electrons to and from the storage capacitor, thereby facilitating read and write operations in the memory device. The memory cells are typically arranged in a number of rows and columns to provide a memory array. Each memory cell in the array is connected to at least one row or “wordline” and at least one column or “bitline.” Generally speaking, the gate terminal of the access device may be coupled to the wordline while at least one of the remaining terminals (e.g. drain/source) is coupled to the bitline. The other terminal (drain/source) may be coupled to the capacitor. When a voltage is applied to the wordline, the gate of the access device opens and charged particles flow from the bitline to the storage device or vice versa, depending on the mode of operation of the memory cell (e.g. read or write).
As the use of semiconductor memory devices has become ubiquitous, many applications have arisen in which the security of the data stored by these same devices is a pressing concern. Frequently, DRAM devices are used to store sensitive data in highly secure applications, because the data stored in such devices must be constantly refreshed in order to maintain the data. Failure to refresh the data in a timely fashion results in eventual loss of the data. However, even when the data is not refreshed, the capacitors that store charge in a DRAM device may continue to do so for some length of time after the latest refresh operation, even if the device is deactivated or powered-down. Though many DRAM devices are specified to retain data for 64 ms, depending upon the operating voltage and temperature, such data is frequently retained for hundreds of milliseconds and may even be retained for times in excess of a full second. While the data may be eventually lost after failure to refresh, this extended period in which the data remains after deactivation of the device presents a security risk that the data will be recovered by reapplying power to the device before the capacitive charge of each memory cell sufficiently dissipates.
This extended storage of data beyond the time of deactivation may be undesirable in a wide range of applications, but is particularly undesirable in military applications and the case of devices intended to be replaced in the field. In this case, if an electronic field replaceable unit (FRU) is removed from a system the data stored in a memory device of that FRU may be accessible if power is restored within a short period of time. Such an event may result in critical data being compromised. Further, some FRUs may actually have capacitive characteristics that provide partial power to the memory device even after the unit has been removed from a power supply, extending the amount of time in which the data could be compromised. In other applications, it may be desirable to destroy or inhibit access to data upon demand, even when the FRU remains connected to the original system.